Leetcode no. 214

214. Shortest Palindrome

Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given "aacecaaa", return "aaacecaaa".

Given "abcd", return "dcbabcd".

public class Solution {
    public String shortestPalindrome(String s) {
        if (s.length() <= 1) return s;
        int end = 0;
        for (int i = s.length()/2+1; i >= 0; i--) {
            int len1 = expand(s, i, i);
            int len2 = expand(s, i, i + 1);
            int len = Math.max(len1, len2);
            if (len > end) {
                end = i + len / 2;
                break;
            }
        }
        if (end==s.length()-1) return s;
        StringBuilder sb= new StringBuilder();
        for (int i = s.length()-1; i > end; i--) {
            sb.append(s.charAt(i));
        }
        sb.append(s);
        return sb.toString();
    }
    private int expand(String s, int left, int right){
        while (left>=0 && right